GTOscott wrote:
1) I don't know if I have the physics correct, but think of it this way...
The air pressure (psi) in your tire acts on the entire inner surface area all the way around the full diameter of the tire (and inner rim surface) to support the bike and rider, thus a skinny tire needs more pressure (psi) to support the bike & rider weight.
1) So lets estimate a 23c x 700c road bike tire has a contact patch with the asphalt of 1.5 square inches with a 150lb rider on board - so this means you have 150 lbs or air supporting the bike and rider (1.5 square inches multiplied by 100 lbs per square inch)
2) Applying similar logic to a 32c x 700c cyclocross tire which has a contact patch of lets say 2.0 square inches x 100 psi = 200 lbs of air supporting the bike and rider which is more than needed and produces a harsh ride
3) The solution for the 32c x 700c cross tire is to run lower pressure (75 psi) to produce the required level of support for the rider (2.0 square inches x 75 psi = 150lbs equivalent to the rider weight)
4) So given the above it takes much less pressure (psi) for a bigger tire to support the bike + rider weight than it does for a skinnier tire
5) This is why a mountain bike tire can be run as low as 20 psi and function properly (fat tire mtb's can successfully run under 10psi!!!!!)
Two tyres at the same pressure have the same size contact patch.
It just that a wider tyre has a wider, shorter shape than a skinny tyre.
This gives less affected sidewall so less rolling resistance.
You only get a larger contact patch if you lower the pressure.